Structural monitoring system

ABSTRACT

A method and device are provided for sensing and collecting structural deformation data on structures over time to permit structural integrity analysis of the structure and determination of structure life expectancy and need for repair or reconstruction of the structure.

FIELD OF THE INVENTION

This invention relates to monitoring and determination of the structuralintegrity of bridges, buildings, transmission towers and the like.Specifically, sensors are attached to the structure to measure variousparameters of structural movement from which a baseline reading isdetermined which may then be compared with later readings from the samestructure in order to determine changes in the natural frequency of thestructure and decay of structural integrity.

BACKGROUND OF THE INVENTION

All man-made structures are subject to stresses and strains during thecourse of their existence which directly affect the structural integrityof the object. In time, the accumulation of stress, strain and vibrationcan result in degradation of materials making up the structure and thepoints of attachment between materials. In the case of a transmissiontower, the force of wind against the structure as well as groundtransmitted vibration can degrade the structural integrity of thetransmission tower. Similar types of stresses and vibration, and inparticular earthquake vibration, can degrade multistory buildings. Oneparticular structure which presents a continuing need for verificationof structural integrity is the bridge or freeway overpass. Thesestructures are continually subjected to repetitive loading, often inexcess of limits, as well as both ground and traffic vibration. Thisresults in deterioration of the bridge structure eventually requiringrepair or replacement of the entire bridge itself.

Presently, however, determinations of structural integrity are generallyestimated by assuming parameters of loading frequency and maximumloading weight in order to make an estimate of bridge life. Theseestimates combined with visual inspection have been utilized todetermine when bridge replacement should occur or when a reduction inthe posted weight limits of a bridge or highway overpass should be made.

For both safety and economy, it is desirable to accurately determine thestructural integrity of a transmission tower, building or bridge. In theparticular case of bridges and highway overpasses, it is necessary forstate and federal governmental entities to detect when bridgereplacement should occur and to be able to forecast such replacements toavoid injuries and for proper financial management. This task iscomplicated by the fact that knowledge of the actual stresses to whichany individual bridge has been subjected is largely unknown.

While estimations of traffic loading and stress and inspections ofbridge exteriors can be made, structural degradation cannot beaccurately predicted or determined as no means exist for documenting thestress history of a particular structure. Such loading history wouldallow engineers to determine whether a particular structure had beensubjected to a greater or lesser degree of loading than estimated. Thisinformation would assist the ability to forecast bridge replacement intwo ways--first, in making an early determination of which bridges arein fact structurally impaired, but not as yet exhibiting visible signsof degradation; and second, in determining which bridges may be selectedfor extended service due to fewer and lighter loading since theirconstruction.

Therefore, it is an object of the present invention to provide a meansfor documenting the loading and vibrational stress applied to astructure during the lifetime of the structure;

Another object of the present invention is to allow comparisons to bemade over the lifetime of a structure with baseline data on theparticular structure in order to determine when sufficient structuraldegradation has occurred to require replacement;

Another object of the present invention is to provide a means ofidentifying which bridges and other structures may receive an extendeduseful life and to distinguish those structures from structures subjectto overloading and a reduced structural lifetime; and

Yet another object of the present invention is to provide a means offorecasting the need to replace bridges and other structures throughaccumulation and comparison of structural stress and loading data onindividual structures.

Other objects and advantages of this invention will become apparent fromthe following description taken in connection with the accompanyingdrawings, wherein is set forth by way of illustration and example, anembodiment of this invention.

SUMMARY OF THE INVENTION

An apparatus and method are provided which enable the long-term analysisand evaluation of the structural integrity of transmission towers,buildings, bridges, freeway overpasses and the like in order to evaluatetheir useful life and present safety status. The method of analysis isaccomplished through the use of an apparatus comprising strain gaugesensors applied to the structure at predetermined locations formeasurement of structure maximum strain and maximum dynamic amplitude.

Data gathered by the sensors is then transmitted to a data recorder forstorage of the collected data or to a computer for data analysis andcomparison. The received data may then be transmitted to a centralfacility for comparison of data with baseline data on the particularstructure and for analysis of structural degradation and safety and forscheduling of structural repair or replacement.

DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic representation of the monitor system showing thesensors attached to the amplification, data recording, transmitting, andpower equipment;

FIG. 2 shows a typical resistance wire strain gauge or sensor which isattached to the structure to be monitored; and

FIG. 3 is a flow chart of the method steps of the monitoring procedure.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Referring to FIG. 1, the equipment modules of structural monitoringsystem 10 are shown in schematic representation. Sensor 12 is attachedto critical areas of the structure to detect strain on the structure atpoints of significant amplitude. Selection of the points of attachmentfor sensor 12 is described hereinafter. Sensor 12 is, in a preferredembodiment, an electrical resistance wire strain gauge 40 (FIG. 2).Strain gauge 40 is comprised of a foil resistance wire element 42attached to an adhesive backing 44. Connector terminals 46 are at eitherend of resistance wire 42 and serve to connect the strain gauge todetector/amplifier 14.

Strain gauge 40 is attached to the structure at selected points in orderto detect the maximum strain endured by the structure during varioustypes of loading. For example, in the case of a bridge it is of interestto collect data on the point of critical strain generated by deflectionof the road bed as traffic passes across it. In this manner, the impactof traffic on the structural integrity of the bridge roadway can bemonitored. Alternatively, where wind induced swaying is a significantconsideration it may be of interest to attach sensor 12 to a bridgesupport tower or other position in order to monitor the effects of suchlateral forces on the structure.

Resistance wire strain gauge 40 operates on the principal that as thestructure responds to an applied load various expansions andcontractions of the structural surface will occur and strain gauge 40attached to the structure also will expand and contract producing achange in the resistance of foil resistance wire 42. This change inresistance of the circuit of sensor 12 is detected by detector-amplifier14. This change in resistance can then be correlated with the loadrequired to produce the degree of deformation in the structure detectedby sensor 12.

Sensor 12 also includes a second strain gauge, like that of FIG. 2,which is placed at a site near or on the structure which is unlikely toreceive significant structural loading and therefore significantexpansion or contraction. This resistance wire temperature sensor 12 isutilized to correct for temperature changes in strain sensor 12 whichdetects the structural strain. It will be appreciated that as sensor 12in most cases will be located outside and subject to extremes oftemperature, it is necessary to correct for the expansion andcontraction of resistance wire 42 which occurs due to temperatureeffects. The second temperature sensor accomplishes this by being linkedthrough a Wheatstone bridge to strain sensor 12. This will thencompensate for changes in temperature of the sensor.

Sensor 12 should be attached to the structure at locations for detectionof maximum information related to structural integrity. This locating ofsensor 12 is accomplished by performing a structural analysis on thespecific structure in order to determine the locations of the structuredisplaying significant motion or actions. This then allows locatingsensor 12 at the portion of the structure most responsive to loading andvibration. Such a structural analysis is a well known technique in theengineering art.

The structural analysis also permits determination of the expectedresponse of the structure through mathematical modeling. The modelingallows a comparison of the accuracy of the signal from sensor 12 withthe calculated expected response from the model. This comparison is alsoconducted in practical fashion by loading the structure with a knownmass and examining the detected response from sensor 12 for comparisonwith the calculated response of the mathematical model of the structure.

Comparison of the expected response with the detected response isnecessary in order to assure that sensor 12 is properly located andinstalled. The output from sensor 12 should be close to that responsecalculated by the structural analysis. If the calculated response anddetected response are not similar, then sensor placement should bereexamined or the initial structural integrity should be reviewed.

The second aspect of a comparison of the calculated response with theinitial detected response is to provide baseline data on the structurefor comparison with future information received from monitor 10. Theexistence of baseline data, taken soon after the completion of thestructure, provides a point of comparison from which a reduction instructural integrity can be referenced.

Another important aspect of obtaining the initial baseline data on thestructure is for analysis of design procedures employed in thedevelopment of the structure. Design procedures that allow for excessivedeflection or stress will be identified and can be eliminated fromfuture structures. As no standard procedures currently exist for suchverification of design procedures the present invention will provide theopportunity for effecting such an analysis in conjunction withmonitoring the safety of the structure over its lifetime.

Once a signal has been detected by amplifier-detector 14 from sensor 12it is transferred to data reader 16. Data reader 16 is intended tocollect and store the information as each sensor 12 responds to stressin the structure. After data has been collected at data reader 16 it maythen be processed on site by controller 18.

The amplified signal from amplifier-detector 14 is read by data reader16. Data reader 16 converts the analog signal from amplifier-detector 14into a digital array. This signal is further converted in order toanalyze two measurements specific to the structure. First the signal isconverted by Fourier transform to provide the frequency domain of thedetected signal. Second the signal is then examined to determine pointsof greatest magnitude. The signal is also examined for other peaks ofsignificance. Once the significant peaks of amplitude and frequencyspectrum have been determined this will then represent a signature forthe particular structure to be utilized in future analysis.

All structural elements in structures vibrate at a frequency peculiar tothat particular structure. This frequency is known in the art as the"natural frequency" of the structure and is a function of both thestructure's weight and stiffness. As the stiffness of the structurebegins to change due to age and deterioration, the natural frequency ofthe structure also will change. A change in the natural frequency of astructure is an important indicator that deterioration of the structureis occurring meriting a more detailed analysis to determine the specificreasons for the cause of the change in the natural frequency. As thepresent invention permits initial establishment of the baseline naturalfrequency of a structure and allows continuous monitoring of thestructure, changes in the natural frequency of the structure can bedetermined by comparison with the baseline data.

After data reader 16 has converted and collected the data resulting fromthe output of amplifier detector 14, controller 18 records the maximumdetected signal magnitude and the frequency at which it was detected.This data is stored in memory for future use. Each controller 18 isassigned an individual code to permit transmission of the data recordedin the controller 18 to a remote central location for analysis. If acontroller 18 is attached to multiple sensors 12, each sensor 12 isconnected to a separate channel within controller 18 and is assigned asecondary access code to allow individual data recording andtransmission.

Each monitor system 10 is equipped with a transmitter 20 and a receiver22 in order that the data stored in controller 18 can be convenientlytransmitted. As many structures utilizing structural monitoring system10 will be in distant locations or locations spread far from one another(i.e., remote transmission towers and bridges along a highway system) itis especially convenient if the data stored in controller 18 can beremotely accessed. Transmitter 20 and receiver 22 accomplish thispurpose by allowing radio communication between controller 18 and theremote central data processing location.

As each controller 18 is assigned an individual code, the centralprocessing facility can transmit a message, including the individualcode of the particular controller and code of the particular sensorchannel, which is then received by antenna 26 and receiver 22. Thisallows initiation of a command to controller 18 and transmitter 20 tobegin sending data via antenna 24 to the remote central processinglocation. This procedure also permits remote monitoring of the operationof monitor 10 and avoids the need to conduct on-site inspections todetermine the operational status of monitor 10 and to retrieve data.Transmitter 20 and receiver 22 can be equipped to function with bothlocal and satellite transmissions depending upon the remoteness of thelocation of the structure. Alternatively, a vehicle can be equipped withthe required communications equipment in order to serve as a mobile datacollection and processing facility.

Monitor 10 is powered, in a preferred embodiment, by battery 28 which isrecharged by solar panel 32. Power lead 30 from battery 28 connects toeach module of monitor 10 in order to power the system.

Once data is received in the remote data processing location, themagnitude of the structure actions can be recorded as well as theparticular frequency at which the maximum of the amplitude spectrumoccurred. These two parameters are then compared with the baseline dataof the structure as well as compared to future data on the structure. Inthis manner a complete structural history can be continually recorded,compared to previous data, and utilized for continuing structuralanalysis and for projecting determinations of needed structurereplacement.

Referring now to FIG. 3 a flow chart of the method steps involved inconducting an analysis with structural monitor 10 is shown. Aspreviously discussed, the first step is selecting the structure formonitoring. Such a structure can be any bridge or building ortransmission tower which is subject to ongoing and undetermined loads orforces which could compromise the structural integrity of the object.After the structure is selected it is then necessary to conduct the stepof analyzing monitor locations 61 in order to situate sensors 12 ofmonitor 10 in positions on the structure demonstrating the greatestdegree of action. This analysis is necessary in order to provideassurance that the forces to which the structure is subjected aredetected, and to insure that the full effect of a force or load on thestructure is determined. After the optimal locations for structuremovement have been determined, monitor 10 is then affixed 62 to thestructure.

Once the monitor is in place on the structure it is then necessary toestablish baseline information regarding the particular structure. Thisis conducted through the step of reviewing the structure with a knownloading 63 in order to determine that the sensor is functioning properlyand essentially to calibrate the sensor output with a known load. Thisstep also provides a determination of the natural frequency of thestructure at a time when it is known to be structurally sound. Thisnatural frequency may then be compared with later collected data fromthe structure to analyze changes in the natural frequency of thestructure indicative of structure degradation. Once the baseline datahas been established, monitor 10 may then be activated to receive data64 from sensor 12. As previously discussed, the received data is thenmanipulated to analyze the frequency of maximum amplitude and thatfrequency at which the maximum amplitude occurred. As the frequency andamplitude information is collected over time it may be transmitted to acentral data processing facility either by manual collection frommonitor 10 or through the use of transmitter receiver devicesincorporated within structural monitor 10. When the data has beenmanipulated and pertinent information obtained on the current naturalfrequency of the structure, this data may then be compared with thebaseline data obtained in the step of reviewing with known loading 63 inorder to provide reports 65 in which the monitor structure is evaluatedand decisions as to structural maintenance and replacement may be made.

It is to be understood that while certain forms of this invention havebeen illustrated and described, it is not limited thereto except insofaras such limitations are included in the following claims and allowablefunctional equivalents thereof.

Having thus described the invention what is claimed as new and desiredto be secured by Letters Patent is as follows:
 1. The method ofmonitoring a structure, a bridge or building for determining thecondition of the structure over time comprising the steps of:Step a:analyzing mathematically the structure to determine a point of asubstantial structural loading action; Step b: attaching a strain gaugeto said analyzed point; Step c: establishing a baseline of strain gaugefrequency output and amplitude output in response to said structuralaction as a result of an applied load; Step d: monitoring said action ofthe structure during use; Step e: recording strain gauge frequencyoutput and amplitude output produced in response to said structuralaction detected by said strain gauge as a result of structure use; Stepf: comparing said recorded frequency and amplitude output with saidbaseline output in order to determine the structural condition of saidstructure with the passage of time, and Step g: repeating steps d-f. 2.The method as claimed in claim 1, wherein said strain gauge is connectedwith a second strain gauge for temperature compensation.
 3. A method fordetermining changes in the structural deterioration of a structurecomprising the steps of:analyzing with a mathematical model thestructure to determine a point of the structure displaying substantialaxial tension-compression in response to loading; attaching a straingauge sensor to said analyzed point; monitoring sensor frequency outputand sensor amplitude output generated by an actual loading of thestructure; determining from said sensor frequency output and said sensoramplitude output a baseline natural frequency of vibration of thestructure at a time soon after sensor attachment; redetermining thenatural frequency of the structure from said sensor frequency output andsaid sensor amplitude output at a time after the determination of saidbaseline; and comparing the redetermined natural frequency of thestructure with said baseline natural frequency of the structure toanalyze deterioration in the structure.
 4. The method as claimed inclaim 3, further comprising the step of transmitting said monitoredfrequency and amplitude of said action to a remote location fordetermination and comparison of said structure natural frequency withsaid baseline.